Adding Oil to the AC System

[shertex]

I have a question about adding oil to the AC system (1991 300D retrofitted to R134a).

To anticipcate some responses (to save the flamers-in-waiting some typing)....

1. Yes, I know that a known leak should be properly repaired.

2. Yes, know that the only way to ensure the correct balance of refrigerant and oil is to evacuate and recharge the system and that I cannot assume that oil and refrigerant are leaking at the same rates....but perhaps even that doesn't ensure a correct balance??

Please humor me anyway.

When I purchased the vehicle in the Spring of 2009, the AC seemed weak so I added some refrigerant. I've since learned that I have a fairly slow leak. Maybe twice a year I add refrigerant and it blows nice and cold.

As a result, I have no real idea how much oil is left in the system. If I weren't so penny wise and pound foolish, I would just evac and recharge the system and get the leak diagnosed....which I may still do.

But if I don't, and if I want to live a bit dangerously, how would I go about making a guesstimate as to how much oil to add? System capacity is 2 oz. (plus 2 lbs. 4 oz. of refrigerant). Just refrigerant with the oil mixed in? Add an ounce every year or two? What's riskier: too much oil or too little?

I figure, on a car this old, the compressor isn't going to last forever anyway. So I'm willing to roll the dice to a degree. I know that what I'm supposed to do is let a professional deal with this....but I just don't want to spend the money.

Thanks.